1. Field of the Invention
The invention relates to cordierite ceramic bodies for use as catalyst carriers, particularly to cordierite bodies, having high thermal shock resistance by virtue of a low coefficient of thermal expansion (CTE), good coatability by a slurry of high surface area activated alumina by virtue of a high level of fine porosity, and a narrow pore size distribution of very fine, generally elongated pores.
2. Discussion of the Related Art
The exhaust gases emitted by internal combustion systems utilizing hydrocarbon fuels, such as hydrocarbon gases, gasoline or diesel fuel, can cause serious pollution of the atmosphere. Among the many pollutants in these exhaust gases are hydrocarbons and oxygen-containing compounds, the latter including nitrogen oxides (NOx) and carbon monoxide (CO). The automotive industry has for many years attempted to reduce the quantities of pollutants from automobile engine systems, the first automobiles equipped with catalytic converters having been introduced in the mid 1970""s.
Cordierite substrates, typically in the form of a honeycomb body, have long been preferred for use as substrates to support catalytically active components for catalytic converters on automobiles, in part due to cordierite ceramics"" high thermal shock resistance. Honeycomb ceramics having a combination of low thermal expansion coefficient, high level of porosity, and high strength are especially attractive for high-performance automotive catalytic converter substrates having thin webs. Demand for cordierite monoliths having very thin webs is increasing in response to legislation requiring higher conversion efficiencies in catalytic converters for the automobile market. Thinner webs reduce the mass of the substrate resulting in faster light-off times. In addition, higher geometric surface areas may be achieved without an increase in the mass of the substrate. Another advantage of thin walled substrates is that a lower backpressure may be achieved.
Despite the advantages of thin-walled cordierite honeycombs, reducing the thickness of the cell walls reduces the strength of the body. This results in problems canning the catalytic converter. If the strength is reduced sufficiently, the canning process can induce a fracture of the substrate material. Therefore, a high level of material strength is required to offset the reduction in geometric strength encountered when the web thickness is very thin. A high porosity is desirable in order to have a high level of coatability of the high surface area alumina washcoat that is applied to the fired substrate. In addition, a high porosity decreases the thermal mass of the substrate and allows for faster lightoff times for the catalytic converter. A low thermal expansion coefficient is important to prevent failures of the substrate material due to thermal shock. Unfortunately, high levels of porosity and low thermal expansion are known to reduce the strength of a cordierite honeycomb. A low thermal expansion is typically achieved by the presence of microcracking within the cordierite matrix that develops during cooling due to thermal expansion anisotropy. The presence of microcracks within the cordierite matrix serve to limit the strength of the body. A high level of porosity in the matrix also limits the strength of the material., since pores serve to decrease the cross sectional area on which a load is applied.
U.S. Pat. No. 4,849,275 (Hamaguchi et al.) discloses a cordierite honeycomb structural body having a porosity of between 30 to 42% with the total volume of all pores in the body consisting of not less than 70%, preferably not less than 80%, of the pores having a diameter between 0.5 to 5.0 xcexcm. U.S. Pat. No. 4,869,944 (Harada et al.) discloses a cordierite body exhibiting a CTE of not greater then 3xc3x9710xe2x88x927/xc2x0 C. in the axial direction and having a porosity of between 30 to 42% and having a porosity in which xe2x89xa740% of the total pores are between 0.5 to 5.0 xcexcm and xe2x89xa630% of the total pores are xe2x89xa710 xcexcm. Although these references each disclose a narrow distribution of fine pores and a sufficiently low CTE, the total porosities disclosed exceed 30%, and they do not disclose, nor do any of the examples exhibit, a narrow pore size distribution of fine enough, elongated and oriented, pores necessary to produce a cordierite body having both a low CTE less than 5xc3x9710xe2x88x927/xc2x0 C. and a sufficiently high strength.
U.S. Pat. No. 4,877,670 (Hamanaka) discloses cordierite bodies having a low CTE less than 10xc3x9710xe2x88x927/xc2x0 C. and a low total volume of pores greater than 5 xcexcm, preferably greater than 2 xcexcm. Although the reference discloses a narrow range of fine pores it does not disclose the combination of a narrow distribution of fine pore sizes, and there is no disclosure regarding the requirement of elongated, oriented pores necessary to ensure the formation of cordierite bodies possessing the property combination of high strength and low CTE of less than 5xc3x9710xe2x88x927/xc2x0 C. 5.
U.S. Patent Application Ser. No. 09/348,307 (Merkel et al.) discloses cordierite bodies exhibiting a CTE of xe2x89xa6xc3x9710xe2x88x927/xc2x0 C. with at least 85% of the total porosity having a mean pore diameter of between 0.5 to 5.0 xcexcm. A second embodiment is disclosed and comprises a cordierite body exhibiting a CTE of between 4 to 6xc3x9710xe2x88x927/xc2x0 C. and having a total porosity of at least 30 vol % with at least 85% of the total porosity having a pore diameter of between 0.5 to 5.0 xcexcm. Once again this reference discloses a narrow distribution fine pores, however there is no disclosure of the elongated, oriented pore structure and narrow distribution of very fine pore sizes that is required to produce cordierite possessing a combination of high strength and low CTE.
It is therefore a principal object of the present invention to provide improved cordierite ceramics, and method for making them, that exhibit a low thermal expansion, high total porosity, high strength, and narrow pore size distribution of small elongated, oriented pores.
The present invention provides for a sintered ceramic substrate and method for making the ceramic substrate, having a primary crystalline phase comprising cordierite and exhibiting a low thermal expansion and high total porosity. The porosity is uniquely comprised of small pores of a narrow size distribution and a generally elongated shape which are believed to contribute substantially to the unexpectedly high strength of these low CTE substrates.
Specifically, the sintered ceramic article of the invention exhibits an average linear coefficient of thermal expansion (25-800xc2x0 C.) below about 5.0xc3x9710xe2x88x927/xc2x0 C., a total porosity between the range of 20% to about 30%. Furthermore, the sintered ceramic article exhibits a pore size distribution such that at least about 86% of pores are of a pore size of less than about 2 xcexcm. Lastly, the ceramic article exhibits an interconnected pore structure with the pores exhibiting a generally elongated shape with the pores being predominately oriented with their long axis in the plane of the webs.
This invention also relates to a method for producing a sintered cordierite ceramic article involving first compounding and plasticizing a cordierite-forming inorganic powder batch comprising a talc having median particle of size less than about 2 xcexcm, preferably a platy talc having a morphology index greater than about 0.75. The batch further comprises at least 4% by weight of the inorganic powder batch mixture of a dispersible Al2O3-forming source having a specific surface area in excess of 50 m2/g and one or more of the components of kaolin, calcined kaolin, silica, and corundum, each having a median particle sizes less than 5 xcexcm.
The plasticized powder batch thus provided is next formed into a green honeycomb by extrusion through a honeycomb extrusion die and the green honeycomb is fired to a temperature and for a time sufficient to convert the green honeycomb into a crystallized cordierite ceramic article having the aforementioned properties.